Many personal and consumer products and packages are made of plastic. Most plastics are thermoplastics. Thermoplastics, when in solid form, melt and flow when they are heated and re-solidify upon cooling. This process is repeatable. On the other hand, some plastics are thermosetting, which means they react or crosslink under heat and pressure and set to form solids. The term “crosslink” means the attachment of two chains of polymer molecules by a bridge formed by an element, a group, or a compound that joins a carbon atom on one chain to a carbon atom on another chain by primary chemical bonds to form a crosslinking network.
Methods for processing either type of plastic, especially thermoplastics, to make personal and consumer products and packaging include injection molding, blow molding, extrusion, thermoforming, and the like. While such processes have been widely used, there are still drawbacks with these present-day processes and with the products made by these processes.
For instance, processing variables have a direct effect on the surface quality and aesthetics of plastic parts manufactured from microcellular foam using injection molding techniques. Gases, for example, are often introduced into the plastic during processing and can detract from the ability to form smooth surfaces in the finished product by contributing to the formation of undesirable surface features. Such undesirable surface features typically occur as swirling patterns or a gritty texture, which place limitations on the manufacturing of the parts. In particular, swirls and gritty textures produce a rough surface quality that not only results in an unappealing finished product, but often undesirably affect the ability to mold parts having thin-walls or similar geometries and/or parts through which channels or the like are desired.
The desire for obtaining a smooth surface, therefore, is one factor in the selection of a molding process using microcellular foam to produce a molded part. There are two major mechanisms that contribute to the formation of rough surfaces in microcellular foam: 1) the gas escapes from the microcellular foam when the microcellular foam is in melt form, or bubbles from the melt overgrow and break; and 2) the bubbles from the melt are sheared in the interface between the mold wall and melt. In either mechanism, the surface of the part produced is compromised via the formation of a defect on the boundary surface thereof.
Moreover, surface roughness is related to the mold filling pattern, and injection processing conditions may influence surface quality significantly. Three different methods exist to smooth the surface for microcellular foamed parts: 1) use of co-injection or gas counter pressure molding processes; 2) use of hot mold surfaces or coated mold surfaces; and 3) surface improvements resulting from processing, mold, material or part design.
Because the current use of microcellular injection molding technology to produce molded parts is less than adequate for some products, measures have been taken to improve surface part quality. These measures include the use of hot, coated surfaces; the use of gas counter-pressure; special developments with regard to the grade of resin used; and/or co-injection. Such measures can be costly, time-consuming, and/or require complicated mold redesigns and operation.